Method for refining molten steel and apparatus therefor

ABSTRACT

A method for refining molten steel by immersing the lower opening end of a cylindrical immersion tube equipped with a lance into the molten steel contained in a ladle, controlling the pressure in the cylindrical immersion tube to a prescribed pressure range to suck up the molten steel, injecting an agitation gas from the bottom of the ladle towards the surface of the sucked-up molten steel, and decarburizing and refining the molten steel under a reduced pressure, characterized in that the method comprising the steps of; controlling the pressure Pt (Torr) in the cylindrical immersion tube so as to satisfy the following formulae (1) and (2), blowing oxygen gas to the surface of the molten steel through the lance, and decarburizing and refining the molten steel under a reduced pressure; 
     
       
           Pt &gt;760−1.297×10 7   /Dc   2   ( 1 ) 
       
     
     
       
           K =1.71× Dl   0.211   ×Dc   0.438   ×Wm   −1.124   ×Qg   0.519   ×Pt   −0.410   &gt;0.046   ( 2 ) 
       
     
     wherein, 
     K: capacity coefficient concerning the decarburizing reaction (l/min.) 
     Dl: inner diameter of the ladle (cm) 
     Dc: circle-reduced diameter of the cylindrical immersion tube (cm) 
     Wm: mass of molten steel per processing (t) 
     Qg: quantity of agitation gas injection (Nm 3 /h.).

TECHNICAL FIELD

This invention relates to a method for refining molten steelinexpensively and efficiently and, more specifically, to a method fordecarburizing, desulfurizing or dephosphorizing molten steelinexpensively and efficiently and a refining apparatus employed forimplementing said method.

BACKGROUND ART

Requirements for steel material properties are becoming more and moredemanding as steel materials are used in more severe environments. Sincesteel materials are widely used in the society in general, they arerequired to be inexpensive, too. For manufacturing steel materialshaving desired properties, it is necessary to lower impurities such asphosphorus, sulfur, carbon, hydrogen, etc. to the least possible amountsat steel refining processes, and it is also important to refine steelinexpensively. In this situation, it is essential to clarify thephysical and chemical fundamentals and principles of steel refiningreactions and develop efficient refining methods and apparatuses basedthereon.

Conventionally, the technical trend of steel refining has been to dividethe refining process into steps so that each of impurities has beenremoved under a condition tailored to facilitate the removal and tocomplete the steel refining through several steps. Technologies based onthis philosophy have come to be widely practiced. For example, widelyemployed is a hot metal treatment process wherein the dephosphorizingtreatment and the decarburizing treatment, which were formerly carriedout using only a converter, have been divided into the dephosphorizingtreatment at the step of molten pig iron and the decarburizing treatmentin a converter.

At the decarburizing treatment in a converter, carbon is removed throughoxidation by injecting oxygen into molten steel (oxidizing refining),but the oxygen is inevitably absorbed in the molten steel.

Oxygen concentration in molten steel becomes high especially whenproducing low carbon steels having a carbon concentration of 0.1% orless: for example, if blowing is stopped at a carbon concentration of0.04%, oxygen content in the molten steel will be 0.05% or so. Thecarbon concentration and the oxygen concentration in molten steel areroughly in inverse proportion to each other and, hence, the lower theend point carbon concentration, the higher the oxygen concentration.

In the meantime, highly formable ultra low carbon steels have come to beused in large quantities especially for exposed panels for automobiles.For producing the ultra low carbon steels, it is necessary to lower thecarbon concentration to a level of 30 ppm or less and, for this purpose,decarburizing treatment is carried out by decompression refining at asecondary refining stage after the decarburization in a converter.

At the present time, when the continuous casting method has becomegeneral, in order to prevent the occurrence of pin holes and breakoutscaused by CO gas generated during casting, it is necessary to removeoxygen absorbed in molten steel by adding a deoxidizing agent, typicallyAl, to molten steel and trapping the oxygen as oxides. When thedeoxidizing agent is entrapped in steel materials, however, it willundesirably cause cracks and defects when they are plated.

Further, the deoxidizing agent remaining in steel materials tends toappear as inclusion-induced defects in the case of low carbon steelsoften used as materials for stamping applications undergoing intensiveworking. A process to produce low carbon steels with low oxygenconcentration, therefore, needs to be developed.

In this respect, a method called the carbon deoxidation method is widelyknown, wherein the oxygen in molten steel is removed in the form of COgas by carbon in the molten steel. In this method a vacuum degassingapparatus equipped with a large evacuator (for example, an RH vacuumdegasser) is generally employed for an effective decarburizing action.

Japanese Unexamined Patent Publication No. S53-16314, for example,discloses a method to produce Al-killed molten steel for continuouscasting use wherein the end point carbon concentration at a converter iscontrolled to 0.05% or more and a degassing treatment is applied using avacuum degasser before deoxidation. By this method, the pressure insidea vacuum tank is controlled within the range of 10 to 300 Torr inaccordance with the progress of decarburization. Further, JapaneseUnexamined Patent Publication No. H6-116626 discloses a decarburizationmethod, with a reduced occurrence of splash, wherein molten steel in aladle with carbon concentration reduced in a converter to 0.1 to 1.0% isdecarburized by immersing a single cylindrical immersion tube into themolten steel and injecting oxygen mixed with an inert gas under apressure of 100 Torr or more.

The methods disclosed in the Japanese Unexamined Patent Publication Nos.S53-16314 and H6-116626, however, employ so-called large decompressionrefining apparatuses. In the method of the Japanese Unexamined PatentPublication No. S53-16314, it is necessary to reduce the pressure to 10Torr or so, and hence a large vacuum degasser such as a vapor jet vacuumpump is indispensable. In the method of the Japanese Unexamined PatentPublication No. H6-116626 wherein oxygen gas mixed with an inert gas isused for decarburization, on the other hand, there is a problem thatexpensive argon gas has to be used since, when inexpensive nitrogen gasis used instead, it is absorbed in steel adversely affecting its agingproperties.

At the present time, when vacuum degassers are widely used for thepurposes of decarburization and dehydrogenation of ultra low carbonsteels, the degassers originally designed for degassing at a high vacuumof 1 Torr or less are often used for the production of low carbonsteels. However, a high decompression refining apparatus such as an RHvacuum degasser (hereinafter sometimes called “an RH refiningapparatus”) has a vacuum tank very large in height and diameter and,consequently, the volume to be evacuated is huge. For this reason, thereare problems of high refining costs due to high unit consumption ofrefractories and high costs of utilities such as steam for a vapor jetvacuum pump required for evacuation.

Another problem is that the construction of a large decompressionrefining apparatus intended for the carbon deoxidation of low carbonsteels is expensive and uneconomical. Further, a high decompressionrefining apparatus is used for producing ultra low carbon steels with acarbon concentration of, for example, 30 ppm or less and, in this case,skulls of a high carbon concentration which adhered onto the inner wallof a vacuum tank when molten steel with a carbon concentration of 0.04%or so, which is a far higher carbon concentration than an ultra lowcarbon steel, is processed, re-melt during the processing of an ultralow carbon steel and become the source of carbon contamination. Thisleads to another problem of longer decarburizing treatment time or noprogress in decarburization. Some RH refining apparatuses are equippedwith an LPG burner for melting and removing the skulls as acountermeasure, but such a countermeasure leads to another problem ofadditional costs for the equipment and the removal operation.

Looking at the desulfurizing treatment of molten steel, it isclassified, generally, into hot metal desulfurization applied in thestate of molten pig iron and molten steel desulfurization applied in thestate of molten steel. As steel materials came to be used in more severeconditions, the required level of steel purity becomes higher. As aconsequence, the application of only the hot metal desulfurization canbe regarded insufficient and the molten steel desulfurization is anindispensable process step. Thus, the development of a method forefficient desulfurization and an apparatus therefor, especially forproducing ultra low sulfur steels having an S concentration of 10 ppm orless, has been required.

As a response, for example, Japanese Unexamined Patent Publication No.S58-37112 proposes a method to immerse an immersion tube (the uplegsnorkel of an RH refining apparatus) equipped with a powder injectionlance into molten steel in a ladle, and to inject a desulfurizing agenttogether with a carrier gas toward the immersion tube.

However, although it is possible to lower the S concentration of moltensteel to 10 ppm or less by this method, a treatment process employingsuch a vacuum degasser has a problem of high operation costs for steam,electricity, etc., because a vacuum degasser such as an RH refiningapparatus has a huge evacuator for maintaining a high vacuum of 1 Torror so. There is another problem of high refractory costs because thevacuum degassing tank has to be very tall and large to cope with theviolent splashing occurring during the course of the processing.

A ladle refining vessel such as an LF is also capable of reducing the Sconcentration of molten steel to a level attainable by the RH process,i.e., 10 ppm or less, but this method has problems of high operationcosts and a low productivity due to the protracted processing time.

As another solution, a desulfurization method has been proposed whereinan immersion tube equipped with a powder injection lance is immersedinto molten steel in a ladle and a desulfurizing agent is injectedtogether with a carrier gas. Although lower in operating cost than thedesulfurizing treatment using an RH apparatus, the proposed methodaccelerates resulfurization by the agitation of slag, which has nodesulfurization capability, on the molten steel surface and it isdifficult to stably produce ultra low sulfur steels with an Sconcentration of 10 ppm or less.

Next, looking at the dephosphorizing treatment of molten steel, thedegassing and dephosphorizing method proposed in Japanese UnexaminedPatent Publication No. S62-205221 can be cited as an example ofconventional methods to dephosphorize molten steel. The method ischaracterized by injecting a dephosphorizing agent in powder from intomolten steel having 100 to 800 ppm of free oxygen through a powderinjection tuyere provided at a lower part of a vacuum degassing tank.However, since a characteristic of the vacuum degasser employed hereinis such that a decarburizing reaction takes place in parallel with thedephosphorizing reaction and the decarburizing reaction proceedspreferentially, there is a shortcoming that the dephosphorizing reactionspeed is lowered.

Facing this situation, Japanese Unexamined Patent Publication No.H2-122013 proposed a new degassing and dephosphorizing method, which wascharacterized in that the degree of vacuum in a degassing tank wascontrolled during the degassing and dephosphorizing process inaccordance with C concentration level of molten steel. Because of acharacteristic of an RH vacuum degasser herein employed, however, thecontrol range of the degree of vacuum where the molten steel processingis viable is usually 150 Torr or less, and the decarburizing reactionproceeds still preferentially at this level of degree of vacuum.Although the proposed method is superior to the method proposed in theJapanese Unexamined Patent Publication No. S62-205221 in terms ofdephosphorizing reaction, it has a problem that a sufficientdephosphorizing speed is not obtained. Another problem is that, in thecase of refining a low carbon steel under the above degree of vacuum, Cconcentration lowers beyond a target concentration according to aproduct standard, and a supplementary addition of carbon-containingalloys is required after dephosphorizing treatment, leading to increasedalloy costs, longer processing time, etc. There is yet another problemwith the method that, since the degree of vacuum is controlled inaccordance with the C concentration level in the molten steel, themolten steel surface in the ladle fluctuates largely, making theoperation difficult.

Further, the problem of high operation costs for steam, electricity,etc. persists with the methods disclosed in the Japanese UnexaminedPatent Publication Nos. S62-205221 and H2-122013, since a huge vacuumdegassing tank such as that of the RH vacuum degasser is employedtherein. These methods also have the problem of high refractory costs,since they have to use a vacuum degassing tank having a sufficientheight to cope with the violent splashing during processing.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above problems ofconventional decarburizing treatments and provide a refining method anda refining apparatus capable of producing low carbon steels efficientlyand inexpensively, and the gist of the present invention is described initems (1) to (3) below.

(1) A method for refining molten steel by immersing the lower openingend of a cylindrical immersion tube equipped with a lance into themolten steel contained in a ladle, controlling the pressure in thecylindrical immersion tube to a prescribed pressure range to suck up themolten steel, injecting an agitation gas from the bottom of the ladletowards the surface of the sucked-up molten steel, and decarburizing andrefining the molten steel under a reduced pressure, characterized inthat the method comprising the steps of; controlling the pressure Pt(Torr) in the cylindrical immersion tube so as to satisfy the followingformulae (1) and (2), blowing oxygen gas to the surface of the moltensteel through the lance, and decarburizing and refining the molten steelunder a reduced pressure;

Pt>760−1.297×10⁷ /Dc ²  (1)

K=1.71×Dl ^(0.211) ×Dc ^(0.438) ×Wm ^(−1.124) ×Qg ^(0.519) ×Pt^(−0.410)>0.046  (2)

wherein,

K: capacity coefficient concerning the decarburizing reaction (l/min.)

Dl: inner diameter of the ladle (cm)

Dc: circle-reduced diameter of the cylindrical immersion tube (cm)

Wm: mass of molten steel per processing (t)

Qg: quantity of agitation gas injection (Nm³/h.).

(2) A method for refining molten steel according to item (1),characterized by receiving, in a ladle, molten steel having a carbonconcentration higher, by 0.03 to 0.06 mass %, than a final target carbonconcentration of 0.02 to 0.06 mass % and decarburizing the steel under areduced pressure.

(3) An apparatus for refining molten steel by providing a cylindricalimmersion tube whose lower opening end is immersed into the molten steelabove a ladle containing the molten steel in a manner to movevertically, sucking up the molten steel into the cylindrical immersiontube, and decarburizing and refining the molten steel under a reducedpressure, characterized by; a lance for blowing oxygen gas to thesurface of the molten steel at the upper portion of the cylindricalimmersion tube, a pressure control means for controlling the pressure Pt(Torr) in the cylindrical immersion tube so as to satisfy the followingformulae (1) and (2) at the upper portion or a side portion of thecylindrical immersion tube, and an agitation gas injection meansprovided at the bottom portion of the ladle for injecting the gas fromthe bottom of the ladle to agitate the molten steel so that said gaspasses through the surface of the molten steel in the cylindricalimmersion tube;

Pt>760−1.297×10⁷ /Dc ²  (1)

K=1.71×Dl ^(0.211) ×Dc ^(0.438) ×Wm ^(−1.124) ×Qg ^(0.519) ×Pt^(−0.410)>0.046  (2)

wherein,

K: capacity coefficient concerning the decarburizing reaction (l/min.)

Dl: inner diameter of the ladle (cm)

Dc: circle-reduced diameter of the cylindrical immersion tube (cm)

Wm: mass of molten steel per processing (t)

Qg: quantity of agitation gas injection (Nm³/h.).

Another object of the present invention is to solve the above problemsof conventional desulfurizing treatments and provide a molten steelrefining method capable of desulfurizing molten steel efficiently andinexpensively, and the gist of the present invention is described initem (4) below.

(4) A method for refining molten steel by immersing the lower openingend of a cylindrical immersion tube equipped with a lance into themolten steel contained in a ladle, controlling the pressure in thecylindrical immersion tube to a prescribed pressure range to suck up themolten steel, injecting an agitation gas from the bottom of the ladletowards the surface of the sucked-up molten steel, and desulfurizing andrefining the molten steel under a reduced pressure, characterized inthat the method comprising the steps of; controlling the pressure in thecylindrical immersion tube to the range of 100 to 500 Torr, controllingthe injection amount of the agitation gas to the range of 0.6 to 3.0Nl/min.·t, blowing a desulfurizing agent in powder form, together with acarrier gas, through the lance to the molten steel surface, anddesulfurizing and refining the molten steel under a reduced pressure.

A further object of the present invention is to solve the above problemsof conventional dephosphorizing treatments and provide a refining methodof low carbon steels capable of dephosphorizing molten steel efficientlyand inexpensively, and the gist of the present invention is described initem (5) below.

(5) A method for refining molten steel by immersing the lower openingend of a cylindrical immersion tube equipped with a lance into themolten steel contained in a ladle, controlling the pressure in thecylindrical immersion tube to a prescribed pressure range to suck up themolten steel, injecting an agitation gas from the bottom of the ladletowards the surface of the sucked-up molten steel, and dephosphorizingand refining the molten steel under a reduced pressure, characterized inthat the method comprising the steps of; controlling the pressure in thecylindrical immersion tube to the range of 100 to 500 Torr, controllingthe injection amount of the agitation gas to the range of 0.6 to 3.0Nl/min.·t, controlling free oxygen in the molten steel to 300 ppm ormore, blowing a dephosphorizing agent in powder form, together with acarrier gas, through the lance to the molten steel surface, anddephosphorizing and refining the molten steel under a reduced pressure.

A yet further object of the present invention is to provide a refiningapparatus for implementing desulfurizing treatment or dephosphorizingtreatment according to the present invention and the gist of the presentinvention is described in item (6) below.

(6) An apparatus for refining molten steel by providing a cylindricalimmersion tube whose lower opening end is immersed into the molten steelabove a ladle containing the molten steel in a manner to movevertically, sucking up the molten steel into the cylindrical immersiontube, and desulfurizing or dephosphorizing and refining the molten steelunder a reduced pressure, characterized by; the cylindrical immersiontube designed so that its height is 3,500 to 7,500 mm and the ratio ofits diameter to the ladle diameter is 0.25 to 0.5, a lance for blowing adesulfurizing or dephosphorizing agent in powder form, together with acarrier gas, to the surface of the molten steel at the upper part of thecylindrical immersion tube, a pressure control means for controlling thepressure in the cylindrical immersion tube to the range of 100 to 500Torr at the upper portion or a side portion of the cylindrical immersiontube, and an agitation gas injection means provided at the bottomportion of the ladle for injecting the gas from the bottom of the ladleto agitate the molten steel so that said gas passes through the surfaceof the molten steel in the cylindrical immersion tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example of an apparatus forimplementing the methods according to the present invention.

FIG. 2 is a graph showing the relationship between the pressure Pt inthe cylindrical immersion tube and the injection amount Qg of theagitation gas in the case that the circle-reduced inner diameter of thecylindrical immersion tube is 80 cm.

FIG. 3 is a graph showing the relationship between the pressure Pt inthe cylindrical immersion tube and the injection amount Qg of theagitation gas in the case that the circle-reduced inner diameter of thecylindrical immersion tube is 150 cm.

FIG. 4 is a graph showing the relationship between the pressure Pt inthe cylindrical immersion tube and the injection amount Qg of theagitation gas in the case that the circle-reduced inner diameter of thecylindrical immersion tube is 200 cm.

FIG. 5 is a graph showing the relationship between the pressure Pt inthe cylindrical immersion tube and the amount Wc of the sucked-up moltensteel.

THE MOST PREFERRED EMBODIMENT

(1) Preferable embodiments of the refining method and the refiningapparatus according to the present invention with regard todecarburization are described hereafter, referring to the drawings.

FIG. 1 shows an apparatus to refine molten steel under a reducedpressure. The following reference numerals in the figure indicate thefollowing apparatuses, respectively: 1 molten steel contained in a ladle2; 3 a vertically movable cylindrical immersion tube installed above theladle 2 so that its lower opening end can be immersed into the moltensteel 1 in the ladle 2; 4 a tuyere installed at the bottom of the ladle2 to inject a molten steel agitation gas; 5 a controller of the degreeof vacuum as a means to control the pressure in the cylindricalimmersion tube 3 to a prescribed value; and 6 a gas blowing or powderblowing lance to blow a gas, or a gas containing a prescribed agent inpowder form, towards the surface of the molten steel 1 in thecylindrical immersion tube 3. When the refining apparatus shown in FIG.1 is used for decarburization, the molten steel 1 is decarburized byblowing a decarburizing gas supplied from a decarburizing gas supplyingsource 7 through the gas blowing lance 6 from the upper part of thecylindrical immersion tube 3 the lower end of which is immersed in themolten steel 1 in the ladle 2 and, at the same time, by injecting amolten steel agitation gas supplied from an agitation gas supplyingsource 8 from the bottom of the ladle 2.

The inventors of the present invention carried out a series oflaboratory scale and real scale tests of decarburization by blowing anappropriate amount of oxygen from the decarburizing gas supplying source7 through the gas blowing lance 6 installed in the cylindrical immersiontube and agitating the molten steel with a bottom-blowing agitation gassupplied from the agitation gas supplying source 8, under differentconditions of the mass of molten steel, the inner diameter of thecylindrical immersion tube, the pressure inside the cylindricalimmersion tube, the gas injection amount, and the ladle inner diameter.As a consequence, the present inventors obtained the results shown inFIGS. 2, 3 and 4. These figures show the points where a final targetcarbon concentration of 0.04% is achieved within 10 min. (a time whichdoes not deteriorate productivity) starting from an initial condition of0.1 mass % of carbon concentration and 0.033 mass % of oxygenconcentration, when decarburizing 300 t or so of molten steel.

From these results, the present inventors worked out formula (2) belowas an expression of the relationship of a capacity coefficient K(l/min.) of the speed of decarburizing reaction defined by equation (3)below with the amount Wm of molten steel per processing, the ladle innerdiameter Dl (cm), the circle-reduced inner diameter Dc (cm) of thecylindrical immersion tube, the injection amount Qg (Nm³/h.) of theagitation gas and the pressure Pt (Torr) in the cylindrical immersiontube.

K=1.71×Dl ^(0.211) ×Dc ^(0.438) ×Wm ^(−1.124) ×Qg ^(0.519) ×Pt^(−0.410)>0.046  (2)

wherein,

K: capacity coefficient concerning the decarburizing reaction (l/min.)

Dl: inner diameter of the ladle (cm)

Dc: circle-reduced diameter of the cylindrical immersion tube (cm)

Wm: mass of molten steel per processing (t)

Qg: quantity of agitation gas injection (Nm³/h.).

K=ln([%C] _(i)/[%C]_(f))/t  (3)

 wherein,

[%C]_(i): carbon concentration before treatment (%)

[%C]_(f): carbon concentration after treatment (%)

t: treatment time (min.)

To advance of the decarburizing reaction, it is necessary to agitateoxygen and molten steel, but it is easier and also preferable in termsof the reaction to blow oxygen to the surface of the molten steel in thecylindrical immersion tube 3 through the gas blowing lance 6 installedinside the cylindrical immersion tube 3. This is because the surface ofthe molten steel in the cylindrical immersion tube 3 is the zone wherebubbles of the injected gas rapidly expand and the agitation is thestrongest. Hence, a high decarburizing efficiency is obtained bysupplying oxygen to the zone.

However, since an excessive supply of oxygen causes a rise in oxygenconcentration in molten steel, it is necessary to choose a suitableinjection amount not to cause the rise. The more gas is blown in fromthe bottom, the better, but too much injection results in fusing damageof the injection nozzle or a porous plug. Thus, it is necessary tochoose a suitable injection amount in consideration of the molten steelmass per processing, the cylindrical immersion tube diameter, the ladlediameter and pressure setting, etc.

More specifically, the values described below are preferable.

(i) The molten steel mass per processing has to be 350 t or less.

This is because, if it exceeds 350 t, the amount of molten steel is toomuch in proportion to the area of reaction surface and it becomesdifficult to complete decarburization within a short time. Too large anamount of molten steel results in a long decarburization time and alarge drop of molten steel temperature, which fact calls for a higherconverter tapping temperature and results in increased refractory costsfor repairs, etc.

(ii) The inner diameter of a ladle has to be 300 cm or more in terms ofcircle-reduced diameter.

When the ladle diameter is small, the speed of decarburizing reactiondecreases to some extent, because the depth of molten steel in a ladlebecomes larger and the static pressure on the bubbles of an injected gasincreases, causing the speed of the decarburizing reaction between theinjected gas and the molten steel to fall. If the amount of theagitation gas is increased to compensate for the fall in the reactionspeed, that will result not only in an increase in the gas cost but alsofusion damage of the tuyere or a porous brick for the gas injection. Ifthe agitation gas injection amount is kept unchanged, thedecarburization time will increase requiring a higher converter tappingtemperature and increased refractory costs, as in the item (i) above.

(iii) The pressure in a cylindrical immersion tube has to be 100 Torr ormore and 500 Torr or less.

A low pressure in the cylindrical immersion tube is advantageous forsecuring the decarburizing reaction speed, but the height of splashbecomes larger, requiring a huge refining apparatus having a height of 7m or more like a conventional RH refining apparatus. When the pressurein the immersion tube exceeds 500 Torr, on the other hand, more gasinjection is required for decarburization, resulting in not only anincrease in the gas cost but also fusion damage of the tuyere or aporous brick for the gas injection. If the agitation gas amount is notincreased, the decarburization time will become longer requiring ahigher converter tapping temperature and increased refractory costs, asin the item (i) above.

(iv) The inner diameter of a cylindrical immersion tube has to be 80 cmor more and 200 cm or less.

If the inner diameter of a cylindrical immersion tube is below 80 cm,the area of the reaction surface becomes small and the decarburizingspeed falls. If the injection amount of the agitation gas is increasedto compensate for the fall in the reaction speed, the height ofsplashing increases, and a problem of fusion damage to the gas injectiontuyere arises. If the agitation gas amount is not increased, thedecarburization time will increase requiring a higher converter tappingtemperature and increased refractory costs, as in the item (i) above.

If the inner diameter of an immersion tube exceeds 200 cm, the amount ofmolten steel sucked up into the cylindrical immersion tube increases,requiring larger equipment to support the increased weight and anincrease in equipment cost as a consequence. Refractory consumption ofthe immersion tube also increases and the costs for its repair alsoincreases.

Under the conditions stated in items (iii) and (iv), the amount ofmolten steel sucked up into the cylindrical immersion tube decreases andthe vertical movement of the vacuum tank becomes easier, requiring onlysimple equipment. This means that an expensive ladle lifting apparatuslike the ones used in the conventional RH vacuum degassers is notnecessary. The splash height can be suppressed by controlling thepressure in the cylindrical immersion tube within the range of 100 to500 Torr. Further, since the inner diameter of the cylindrical immersiontube is 80 to 200 cm, smaller than conventional decompression refiningapparatuses, unit consumption of the refractory is smaller and itsrepair work easier.

A sufficient gas injection amount can be secured with the one porousbrick conventionally used in a ladle, and it is not necessary to add anew gas injection hole or use a special porous brick or lance for thedecarburization processing according to the present invention.

Further, when producing a low carbon steel having a final target carbonconcentration of 0.02 to 0.06 mass %, efficient refining is possible bystopping the converter blowing at a carbon concentration higher, by 0.03to 0.06 mass % or so, than a target carbon concentration and thendecarburizing the steel under a reduced pressure using the refiningmethod and apparatus according to the present invention. Molten steel,lower in carbon concentration than that obtainable by the conventionaldecarburization processing by a converter to hit the target carbonconcentration in one step, can thus be obtained more inexpensively.

(2) Preferable embodiments of the refining method and the refiningapparatus according to the present invention with regard todesulfurization are described hereafter referring to the drawings.

A refining apparatus of the same type as shown in FIG. 1 is used. In therefining apparatus shown in FIG. 1, the degree of vacuum inside thecylindrical immersion tube 3 is controlled within the range of 100 to500 Torr by the controller of the degree of vacuum 5. The molten steel 1is desulfurized by controlling the degree of vacuum inside thecylindrical immersion tube 3 within the range of 100 to 500 Torr asstated above and the amount of molten steel agitation gas injectedthrough the tuyere 4 within the range of 0.6 to 3.0 Nl/min.·t. Thedesulfurization processing according to the present invention describedabove is based on the finding that, for producing ultra low carbonsteels, it is important to intensify agitation of (1) the portion ofmolten steel where powder is injected and (2) the entire molten steel ina ladle. When a desulfurizing agent is injected into molten steel, adesulfurizing reaction proceeds while the agent is suspended in themolten steel. Here, if agitation is intensified in the portion where thepowder is injected, that is, if molten steel is agitated especiallyunder a reduced pressure, the agitation by gas expansion under thereduced pressure is added to the agitation by the agitation gas alone,resulting in an acceleration of the desulfurizing reaction, comparedwith that under normal pressure, due to the intensified agitation.Removal of locally desulfurized molten steel from the powder injectedportion and a quick supply of fresh molten steel to that portion by theintensified agitation prevent the desulfurization reaction rate frombeing determined by the movement velocity of S in the molten steel tothe desulfurizing reaction surface.

By the refining method of the present invention, as described above, themolten steel 1 is desulfurized under the conditions of a degree ofvacuum in the cylindrical tube 3 of 100 to 500 Torr and an injectionamount of the gas for agitating molten steel of 0.6 to 3.0 Nl/min.·t.The reason why the degree of vacuum inside the cylindrical tube 3 iscontrolled within the range of 100 to 500 Torr is as follows. If thedegree of vacuum exceeds 500 Torr, the steel agitation at the powderinjected portion becomes insufficient making it impossible to lower theS concentration in the molten steel to 10 ppm or less. When the degreeof vacuum is below 100 Torr, on the other hand, a huge vacuum degassingtank of a sufficient height is required to cope with violent splashingduring the desulfurization processing, resulting in undesirably highoperation costs.

Further, the reason why the injection amount of the gas for agitatingmolten steel is controlled to the range of 0.6 to 3.0 Nl/min.·t is asfollows. When the gas is injected at a rate exceeding 3.0 Nl/min.·tthrough a commonly used porous brick, fusion damage to the brick is soadvanced that its service life becomes short and, besides, slag on themolten steel surface is greatly stirred by strong rocking motion of themolten steel in the ladle, making it impossible to decrease Sconcentration in the molten steel to 10 ppm or lower. If the gasinjection amount is below 0.6 Nl/min.·t, mixing of the entire moltensteel becomes too weak, making it impossible to decrease S concentrationin the molten steel to 10 ppm or lower.

For more efficient desulfurizing treatment, a cylindrical immersion tube3 has to be so designed that its height is 3,500 to 7,500 mm and theratio of its diameter to the ladle diameter is 0.25 to 0.5. The reasonfor this is as follows: when the height of the cylindrical immersiontube 3 is below 3,500 mm and the ratio of its diameter to the ladlediameter is below 0.25, the yield of molten steel is lowered and therefining operation becomes unstable due to an increase in the amount ofskulls sticking onto the inner wall of the cylindrical immersion tube asa result of splash during the processing; when the height of thecylindrical immersion tube 3 exceeds 7,500 mm and the ratio of itsdiameter to the ladle diameter exceeds 0.5, the size of the entireapparatus becomes nearly as large as a vacuum degasser such as an RHrefining apparatus, resulting in undesirably high operation costs.

(3) Preferable embodiments of the refining method and the refiningapparatus according to the present invention with regard todephosphorization are described hereafter referring to the drawings.

A refining apparatus of the same type as shown in FIG. 1 is used. In therefining apparatus shown in FIG. 1, the degree of vacuum inside thecylindrical immersion tube 3 is controlled within the range of 300 to500 Torr by the controller of the degree of vacuum 5. The molten steel 1is dephosphorized by controlling the degree of vacuum inside thecylindrical immersion tube 3 to within the range of 300 to 500 Torr asstated above, the amount of molten steel agitation gas injected throughthe tuyere 4 to within the range of 0.6 to 3.0 Nl/Nl/min.·t, and freeoxygen in the molten steel to 300 ppm or more. The dephosphorizationprocessing according to the present invention as described above isbased on the finding that it is important to intensify agitation of (1)the portion of molten steel where powder is injected and (2) the entiremolten steel in a ladle. When a dephosphorizing agent is injected intomolten steel, dephosphorizing reaction proceeds while the agent issuspended in the molten steel. Here, if steel agitation is intensifiedin the portion where the powder is injected, that is, if molten steel isagitated especially under a reduced pressure, the agitation by gasexpansion under the reduced pressure is added to the agitation by theagitation gas alone, resulting in an acceleration of the dephosphorizingreaction, compared to that under the normal pressure, due to theintensified agitation.

By the refining method of the present invention, as described above, themolten steel is dephosphorized under the conditions of a degree ofvacuum in the cylindrical tube 3 of 300 to 500 Torr, an injection amountof the gas for agitating molten steel of 0.6 to 3.0 Nl/min.·t, and freeoxygen in the molten steel of 300 ppm or more. The reason why the degreeof vacuum in the cylindrical tube 3 is controlled within the range of300 to 500 Torr is as follows. If the degree of vacuum exceeds 500 Torr,the steel agitation at the powder injected portion is insufficient andthe dephosphorizing reaction becomes very slow. When the degree ofvacuum is below 300 Torr, on the other hand, the decarburizing reactionproceeds preferentially causing undesirable effects such as slowing downof the dephosphorizing reaction, a supplementary addition ofcarbon-containing alloys after the dephosphorizing treatment due toover-reduction of C concentration of the molten steel beyond the Cconcentration by the product standard, and an increase in operationcosts because of a huge vacuum degassing tank of a sufficient heightrequired for coping with violent splashing occurring during thedephosphorizing treatment.

Further, the reason why the amount of the gas for agitating molten steelis controlled within the range of 0.6 to 3.0 Nl/min.·t is as follows.When the gas is injected at a rate exceeding 3.0 Nl/min.·t through acommonly used porous brick, fusion damage to the brick becomes soadvanced that its service life becomes short and, besides, a rockingmotion of the molten steel in the ladle becomes too strong to securestable operation.

If the gas injection amount is below 0.6 Nl/min.·t, mixing of the entiremolten steel becomes too weak and the dephosphorizing reaction slowsdown remarkably. The reason why free oxygen in the molten steel has tobe kept at 300 ppm or more is that, when the free oxygen is below 300ppm, the dephosphorizing reaction slows down remarkably due toinsufficient free oxygen.

For more efficient dephosphorizing treatment, the cylindrical immersiontube 3 has to be so designed that its height is 3,500 to 7,500 mm andthe ratio of its diameter to the ladle diameter is 0.25 to 0.5. Thereason for this is as follows: when the height of the cylindricalimmersion tube is below 3,500 mm and the ratio of the immersion tubediameter to the ladle diameter is below 0.25, the molten steel yield islowered and the refining operation becomes unstable due to an increasein the amount of skulls sticking onto the inner wall of the cylindricalimmersion tube as a result of splash during the processing; when theheight of the cylindrical immersion tube 3 exceeds 7,500 mm and theratio of its diameter to the ladle diameter exceeds 0.5, the size of theentire apparatus becomes nearly as large as a vacuum degasser such as anRH refining apparatus, resulting in undesirably high operation costs.

EXAMPLES Example I

This example relates to decarburizing treatment.

For the purpose of producing a low carbon steel having a final carbonconcentration of 0.04%, Inventive Example 1 in Table 1 was prepared asfollows: 292 t of molten steel was tapped to a ladle from a converter,after stopping blowing, at a carbon concentration of 0.07%, and it thenunderwent a decarburizing treatment for 9 min. using a refiningapparatus shown in FIG. 1 with an inner diameter of the cylindricalimmersion tube of 165 cm, a ladle inner-diameter of 400 cm, a pressurein the cylindrical immersion tube of 300 Torr and a bottom blowing gasamount of 37 Nm³/h. The molten steel decarburized under the abovecondition was then deoxidized with an aluminum addition to finallyobtain a molten steel having a carbon concentration of 0.04%. The yieldof aluminum at this treatment was 93% and that of manganese ore at theconverter was 65%.

Inventive Example 2 in Table 1 was prepared as follows: 260 t of moltensteel was tapped to a ladle from a converter, after stopping blowing, ata carbon concentration of 0.08%, and it then underwent a decarburizingtreatment for 12 min. with oxygen blowing through the top blowing lanceunder the condition of an inner diameter of the cylindrical immersiontube of 86 cm, a ladle inner diameter of 400 cm, a pressure in thecylindrical immersion tube of 200 Torr and a gas injection amount of 40Nm³/h., to achieve a final carbon concentration of 0.04%. The steel thusobtained was finally deoxidized with an aluminum addition. The yield ofaluminum at this treatment was 94% and the reduction yield of manganeseore at the converter was 68%.

Comparative Example 1 in Table 1 was prepared by decarburizing 290 t ofmolten steel melted in a converter having a carbon concentration of0.07%. The decarburization condition was as follows: a ladle innerdiameter of 250 cm, an inner diameter of the cylindrical immersion tubeof 70 cm, and a gas injection amount of 50 Nm³/h. In this case nopressure controller was used and the refining proceeded under the normalatmospheric pressure for 20 min., resulting in a carbon concentrationreduction only to 0.05% and, adversely, a rise in oxygen concentration.At an aluminum addition thereafter for deoxidation, the yield ofaluminum was as low as 68%.

Comparative Example 2 in Table 1 is an example of a case that aconventional RH vacuum degasser was used and it was prepared bydecarburizing a molten steel melted in a converter to a carbonconcentration of 0.08%. After a decarburizing treatment for 6 min., acarbon concentration of 0.04% was attained. More steam and electricitywere consumed in this case than in the examples of the presentinvention.

Comparative Example 3 in Table 1 is an example of a case that carbonconcentration was brought down to 0.04% through decarburization in onestep in a conventional converter. In this case both the manganese yieldand the aluminum yield were low.

TABLE 1 Secondary refining After refining by converter Molten steelchemical composition Chemical composition of molten steel FeO Mn afterdecarburization Al Electricity Steam (%) in slag yield (%) yieldconsumption consumption C Si Mn P S O (%) (%) C Si Mn P S O (%) (kWh/l)(kg/l) Inventive 0.07 tr 0.24 0.015 0.011 0.034 13.7 65 0.04 tr 0.240.015 0.011 0.034 93 0.11 0   Example 1 Inventive 0.08 tr 0.25 0.0140.012 0.032 12.5 68 0.04 tr 0.25 0.014 0.012 0.032 94 0.13 0   Example 2Comparative 0.07 tr 0.24 0.016 0.012 0.032 12.2 64 0.05 tr 0.18 0.0160.012 0.042 68 0.10 0   Example 1 Comparative 0.08 tr 0.26 0.015 0.0130.031 11.8 65 0.04 tr 0.26 0.015 0.013 0.031 88 8.0  3.2 Example 2Comparative 0.04 tr 0.19 0.015 0.013 0.056 12.1 38 0.04 tr 0.19 0.0150.013 0.056 62 0   0   Example 3

Example II

Molten steel 1 having 26 ppm of S concentration was desulfurized using arefining apparatus shown in FIG. 1 as a desulfurizing reaction vessel. Acylindrical immersion tube 3 immersed in a ladle 2 had an inner diameterof 1.5 m and a height of 4.5 m, and the pressure inside the tube 3 waskept at 200 Torr by a controller of the degree of vacuum 5. The moltensteel 1 was agitated with Ar gas, for agitating the molten steel,injected through a tuyere 4 at the bottom of the ladle 2 at a rate of1.8 Nl/min.·t and, in parallel, it was desulfurized with a desulfurizingagent in powder form injected at a rate of 5 kg/t together with acarrier gas through a powder injection lance 6. The result is shown inTable 2. It was confirmed that S concentration [S] in the molten steelwas reduced from 26 ppm before the desulfurization to 5 ppm thereafterand that the desulfurization proceeded efficiently and with a lowoperating cost.

Table 2 also shows comparative examples: Comparative Example 1 is a casethat desulfurization was done using a conventional RH vacuum degasserinjecting a desulfurizing agent in powder form at a rate of 4.5 kg/t. Inthis case, the [S] concentration was reduced from 28 ppm before thedesulfurization to 6 ppm thereafter, but with a very high operatingcost.

Comparative Example 2 in Table 2 is a case that the desulfurizingreaction vessel according to the present invention was used, injecting adesulfurizing agent in powder form at a rate of 3 kg/t together with acarrier gas through a lance, but under the atmospheric pressure (760Torr) without using a controller of the degree of vacuum. In this case,the [S] concentration was reduced from 31 ppm before the desulfurizationonly to 26 ppm thereafter, failing to attain a target of [S]≦10 ppm.

TABLE 2 Desulfur- Degree [S] before [S] after Amount of izing ofdesulfur- desulfur- desulfurizing reaction vacuum ization ization agentvessel (Torr) (ppm) (ppm) (kg/t) Inventive The one as 200 26  5 5Example shown in FIG. 1 Com- RH  1 28  6 4.5 parative Example 1 Com- Theone as 760 31 26 3 parative shown in Example 2 FIG. 1

Example III

Molten steel 1 having 340 ppm of free oxygen and 96 ppm of Pconcentration was dephosphorized using a refining apparatus shown inFIG. 1 as a dephosphorizing reaction vessel. A cylindrical immersiontube 3 immersed in a ladle 2 had an inner diameter of 1.5 m and a heightof 4.5 m, and the pressure inside the cylindrical immersion tube 3 waskept at 350 Torr by a controller of the degree of vacuum 5. The moltensteel 1 was agitated with Ar gas, for agitating molten steel, injectedthrough a tuyere at the bottom of the ladle 2 at a rate of 1.8 Nl/min.·tand, in parallel, a dephosphorizing agent in powder form was injected ata rate of 4 kg/t together with a carrier gas through a powder injectionlance 6. The result is shown in Table 3. It was confirmed that Pconcentration [P] in the molten steel was reduced from 96 ppm before thedephosphorization to 22 ppm thereafter and that the treatment proceededefficiently and with a low operating cost.

Table 3 also shows comparative examples: Comparative Example 1 is a casethat a conventional RH vacuum degasser was used with a dephosphorizingagent in powder form injected at a rate of 4 kg/t. In this case, [P]concentration was reduced from 100 ppm before the desulfurization to 25ppm thereafter, but with a very high operating cost.

Comparative Example 2 in Table 3 is a case that a dephosphorizingreaction vessel according to the present invention was used with thedephosphorizing agent in powder form injected at a rate of 4 kg/ttogether with a carrier gas through a lance to treat a molten steelhaving 194 ppm of free oxygen. In this case, [P] concentration wasreduced from 110 ppm before the dephosphorization to 95 ppm thereafter,but at a very slow dephosphorization speed.

Comparative Example 3 in Table 3 is a case that the dephosphorizingreaction vessel according to the present invention was used with thedephosphorizing agent in powder form injected at a rate of 4 kg/ttogether with a carrier gas through a lance, but under the atmosphericpressure (760 Torr) without using a controller of the degree of vacuum.In this case, [P] concentration was reduced from 92 ppm before thedephosphorization to 83 ppm thereafter, but at a very slowdephosphorization speed.

TABLE 3 [P] De- before [P] after Amount of phosphor- Degree dephos-dephos- dephos- izing of Free phor- phor- phorizing reaction vacuumoxygen ization ization agent vessel (Torr) (ppm) (ppm) (ppm) (kg/t)Inventive The one as 350 340  96 22 4 Example shown in FIG. 1 Com- RH 80 400 100 25 4 parative Example 1 Com- The one as 350 190 110 95 4parative shown in Example 2 FIG. 1 Com- The one as 760 450  92 83 4parative shown in Example 3 FIG. 1

INDUSTRIAL AVAILABILITY

The method and apparatus to refine molten steel according to the presentinvention are capable of decarburizing, desulfurizing or dephosphorizingmolten steel, especially that of low carbon steels, efficiently and witha low operating cost. Thus the present invention provides a usefulrefining method of steel production and an apparatus therefor.

What is claimed is:
 1. A method for refining molten steel by immersingthe lower opening end of a cylindrical immersion tube equipped with alance into the molten steel contained in a ladle, controlling thepressure in the cylindrical immersion tube to a prescribed pressurerange to suck up the molten steel, injecting an agitation gas from thebottom of the ladle towards the surface of the sucked-up molten steel,and decarburizing and refining the molten steel under a reducedpressure, characterized in that the method comprising the steps of;controlling the pressure Pt (Torr) in the cylindrical immersion tube soas to satisfy the following formulae (1) and (2), blowing oxygen gas tothe surface of the molten steel through the lance, and decarburizing andrefining the molten steel under a reduced pressure; Pt>760−1.297×10⁷ /Dc²  (1) K=1.71×Dl ^(0.211) ×Dc ^(0.438) ×Wm ^(−1.124) ×Qg ^(0.519) ×Pt^(−0.410)>0.046  (2)  wherein, K: capacity coefficient concerning thedecarburizing reaction (l/min.) Dl: inner diameter of the ladle (cm) Dc:circle-reduced diameter of the cylindrical immersion tube (cm) Wm: massof molten steel per processing (t) Qg: quantity of agitation gasinjection (Nm³/h.).
 2. A method for refining molten steel according toclaim 1, characterized by receiving, in a ladle, molten steel having acarbon concentration higher, by 0.03 to 0.06 mass %, than a final targetcarbon concentration of 0.02 to 0.06 mass % and decarburizing the steelunder a reduced pressure.
 3. A method for refining molten steel byimmersing the lower opening end of a cylindrical immersion tube equippedwith a lance into the molten steel contained in a ladle, controlling thepressure in the cylindrical immersion tube to a prescribed pressurerange to suck up the molten steel, injecting an agitation gas from thebottom of the ladle towards the surface of the sucked-up molten steel,and desulfurizing and refining the molten steel under a reducedpressure, characterized in that the method comprising the steps of;controlling the pressure in the cylindrical immersion tube to the rangeof 100 to 500 Torr, controlling the injection amount of the agitationgas to the range of 0.6 to 3.0 Nl/min.·t, blowing a desulfurizing agentin powder form, together with a carrier gas, through the lance to themolten steel surface, and desulfurizing and refining the molten steelunder a reduced pressure.
 4. A method for refining molten steel byimmersing the lower opening end of a cylindrical immersion tube equippedwith a lance into the molten steel contained in a ladle, controlling thepressure in the cylindrical immersion tube to a prescribed pressurerange to suck up the molten steel, injecting an agitation gas from thebottom of the ladle towards the surface of the sucked-up molten steel,and dephosphorizing and refining the molten steel under a reducedpressure, characterized in that the method comprising the steps of;controlling the pressure in the cylindrical immersion tube to the rangeof 100 to 500 Torr, controlling the injection amount of the agitationgas to the range of 0.6 to 3.0 Nl/min.·t, controlling free oxygen in themolten steel to 300 ppm or more, blowing a dephosphorizing agent inpowder form, together with a carrier gas, through the lance to themolten steel surface, and dephosphorizing and refining the molten steelunder a reduced pressure.
 5. An apparatus for refining molten steel byproviding a cylindrical immersion tube whose lower opening end isimmersed into the molten steel above a ladle containing the molten steelin a manner to move vertically, sucking up the molten steel into thecylindrical immersion tube, and decarburizing and refining the moltensteel under a reduced pressure, characterized by; a lance for blowingoxygen gas to the surface of the molten steel at the upper portion ofthe cylindrical immersion tube, a pressure control means for controllingthe pressure Pt (Torr) in the cylindrical immersion tube so as tosatisfy the following formulae (1) and (2) at the upper portion or aside portion of the cylindrical immersion tube, and an agitation gasinjection means provided at the bottom portion of the ladle forinjecting the gas from the bottom of the ladle to agitate the moltensteel so that said gas passes through the surface of the molten steel inthe cylindrical immersion tube; Pt>760−1.297×10⁷ /Dc ²  (1) K=1.71×Dl^(0.211) ×Dc ^(0.438) ×Wm ^(−1.124) ×Qg ^(0.519) ×Pt^(−0.410)>0.046  (2)  wherein, K: capacity coefficient concerning thedecarburizing reaction (l/min.) Dl: inner diameter of the ladle (cm) Dc:circle-reduced diameter of the cylindrical immersion tube (cm) Wm: massof molten steel per processing (t) Qg: quantity of agitation gasinjection (Nm³/h.).
 6. An apparatus for refining molten steel byproviding a cylindrical immersion tube whose lower opening end isimmersed into the molten steel above a ladle containing the molten steelin a manner to move vertically, sucking up the molten steel into thecylindrical immersion tube, and desulfurizing or dephosphorizing andrefining the molten steel under a reduced pressure, characterized by;the cylindrical immersion tube designed so that its height is 3,500 to7,500 mm and the ratio of its diameter to the ladle diameter is 0.25 to0.5, a lance for blowing a desulfurizing or dephosphorizing agent inpowder form, together with a carrier gas, to the surface of the moltensteel at the upper part of the cylindrical immersion tube, a pressurecontrol means for controlling the pressure in the cylindrical immersiontube to the range of 100 to 500 Torr at the upper portion or a sideportion of the cylindrical immersion tube, and an agitation gasinjection means provided at the bottom portion of the ladle forinjecting the gas from the bottom of the ladle to agitate the moltensteel so that said gas passes through the surface of the molten steel inthe cylindrical immersion tube.